Since the ships are accelerated and decelerated by lasers on the earth side, that constrains the time at earth some. If I remember correctly the ships accelerate at 1.5 g so that will take just under half a year to reach the 0.7c crusing speed. Guessing that there is just one earth side laser system it can either be speeding up or slowing down one ship. That means one ship incoming and one leaving per year. Each needs almost six months of laser time. So a ship comes back and as soon as it arrives another is sent to Pandora. The incoming ship has one year for refitment and can be sent after the next incoming ship arrives.